why can a power supply never supply to much amperage but can supply to much voltage and fry your "thing"? If a device only pulls the amount of amps it needs why isnt the same true for volts?
The simple Ohm's law formula explains the relationship and interdependence between voltage, current, and load resistance. Finding a short primer on Ohms Law should help you on a journey of understanding on this topic.
and my second question, whats the cheapest way to step down 12v DC from a battery to 6v DC with a max draw of 4amps? I could use a through hole chip or a laptop charger type thing with an In/Out connector.
That's a max power output of 24 watts, not something you are going to get by real cheaply with. A switching voltage regulator module is probably the best avenue to look for.